THE GEOMORPHIC EXPRESSION OF NORTH …...THE GEOMORPHIC EXPRESSION OF NORTH VERSUS SOUTH POLAR...

THE GEOMORPHIC EXPRESSION OF NORTH VERSUS SOUTH POLAR LAYEREDOUTCROPS ON MARS AT METER TO DECAMETER SCALES. M. C. Malin and K. S. Edgett,Malin Space Science Systems, P. O. Box 910148, San Diego, CA 92191-0148, USA.

Synopsis: Mars Orbiter Camera (MOC) imagesacquired by the Mars Global Surveyor (MGS) since1997 show that the layered outcrops and general geo-morphic character of the two martian polar regions areradically different. These differences occur at scales ofhundreds to thousands of meters, and are likely to indi-cate differences in depositional, erosional, and climatehistory between the two poles over long periods oftime (perhaps millions of years).

Observations: MGS began orbiting Mars inSeptember 1997. As of April 2000, hundreds of MOCimages cover portions of the north and south polarresidual caps and surrounding layered terrain at scalesranging from 1.4 to 15 m/pixel under illumination andseasonal conditions that include winter, spring, andsummer in the south and spring, summer, and autumnin the north. These images reveal major differences inthe morphology of the two residual cap surfaces andneighboring polar layered terrain.

Residual Caps. The two residual polar caps differgeomorphically from each other as well as from therest of the martian surface. Initial observations weresummarized by Thomas et al. [1]. What is most strik-ing is the “swiss cheese”-like morphology of much ofthe south residual cap (Fig. 1a). No surfaces elsewhereon Mars have this pattern of circular depressions andsags among arcuate-scarped mesas. The north residualcap, in contrast, is largely flat at the hectometer scalebut exhibits abundant decameter-scale pits (Fig.1b)—these pits have not been seen on the south resid-ual cap. The “swiss cheese” terrain in the south is along-term geologic feature, not an expression of sea-sonal wintertime frost. In summer, this terrain hasbeen observed in a state of defrosting such that the ar-cuate scarps (Fig. 2)—some of them exhibiting alter-nating light- and dark-toned layers (Fig. 3)—are darkrelative to their winter/spring appearance.

Dark Lanes. Like everything else about the two po-lar caps, the dark “lanes” that occur in each residual capdiffer between the poles. In the north, the equator-wardslopes exhibit banding and ridge-and-trough morpholo-gies indicating layers of material of differing thickness,albedo, and resistance to erosion; while the polewardslopes are usually mantled and show no evidence oflayering (Fig. 4a). Dark lanes in the south polar regionshow a semi-symmetric pattern of somewhat stair-stepped and folded/wrinkled layers on either side of atrough; these typically bound a rugged, medial ridge(Fig. 4b).

Polar Layered Terrain. The layered units that sur-round each residual polar cap (and which are more ex-tensive in the southern hemisphere) also exhibit differ-ent geomorphic expressions between the north andsouth poles [2]. Though there are exceptions, in gen-eral the layers in the north tend to be expressed in thewalls of the ‘dark lanes’ as a sequence of ridges anddepressions while southern layers tend to have a stair-stepped or cliff-bench topography (Fig. 5). Layeredoutcrops in both hemispheres exhibit considerable evi-dence for a diversity of physical properties, for examplelayers that are expressed as ridges must consist of mate-rial that is more resistant to erosion than those ex-pressed as depressions. When exposed at the surface inplanimetric form, some layers in the south polar regiondevelop spidery networks of closed gully-like depres-sions that are confined to that specific layer (Fig. 6a);others have a smooth-textured surface (e.g., bright me-sas in Fig. 6b); still others are rugged or rough at me-ter scales (Fig. 6c). Low albedo dunes superposed uponlayered units in both hemispheres (Fig. 6d) indicatethat the layers form a hard substrate across which sandcan be transported via saltation—this observationmight indicate that the upper surfaces of the polar lay-ered units are not mantled by thick dust as has beeninterpreted on the basis of thermal inertia mapping [3].

Conclusions. MGS MOC images are giving usa new perspective on the geomorphology and geologichistory of Mars. The observations presented here leadto two basic conclusions:

(1) The polar layered units include individual layersthat have properties that differ from other layers. Thismeans that individual layers probably record changes inmaterials deposited and processes that have operatedover geologic time scales.

(2) The polar layered materials both on and off theresidual caps are different in the north relative to thesouth, and different as well from other parts of Mars.The two polar regions have likely been quite differentfrom each other—in terms of climate and materialsdeposited—for a very long time.

References. [1] Thomas P. C. et al. (2000) Na-ture, 404, 161–164. [2] Malin M. C. and Edgett K. S.(2000) LPS XXXI, #1055. [3] Paige D. A. and K. D.Keegan (1994) JGR, 99, 25993–26013.

POLAR LAYER GEOMORPHIC EXPRESSION: M. C. Malin and K. S. Edgett

All illuminated from upper left. Figure 1: Typical surface mor-phology of south (A) and north (B) residual caps; (A) M09-05133;87.1°S, 95.4°W. (B) CAL-00433; 86.9°N, 207.5°W. Figure 2: De-frosting south residual cap (A) at Ls 254° (M10-00093) and (B) Ls320° (M13-01121); 86.5°S, 344.7°W. Figure 3: Banded slopes onmesa in south residual cap at Ls 306°, M12-02295; 87.0°S,341.2°W. Figure 4: Dark lane crossings in (A) north polar cap,M00-02072; 86.0°N, 258.4°W and (B) south polar cap, M08-05817;86.8°S, 350.8°W. Figure 5: Typical layer expressions (A) stair-

stepped in south, M13-00253; 87.0°S, 187.7°W and (B) ridged-and-troughed in the north, SP2-52406; 86.3°N, 199.2°W. Figure 6:South polar layered unit surfaces in planview; (A) gullied surface,M12-00228; 77.9°S, 193.0°W; (B) smooth, mesa-forming units(light-toned), M09-06148; 79.6°S, 298.3°W; (C) typical ruggedpolar surface, M12-00563, 86.7°S, 264.9°N; (D) eolian dunes trav-eling across layered terrain surface, M12-00230; 76.7°S, 195.4°W.